There are many known olefin polymers that are generally suitable for making extrusion coated or laminated substrates such as paperboard. Although Ziegler catalyzed polymers such as heterogeneously branched linear low density ethylene polymers and substantially linear ethylene polymers have been described as suitable for extrusion coating, laminating and extrusion cast film purposes, high pressure free-radical initiated ethylene polymers such as low density polyethylene (LDPE), ethylene-acrylic acid (EAA) copolymers and ethylene-vinyl acetate (EVA) copolymers continue to be the dominant resins of choice for various extrusion coating and laminating applications. While any of the aforementioned ethylene polymers can be conveniently extrusion coated or laminated onto a substrate to prepare an initially pinhole-free coating or laminate, for many applications, when the coated or laminated substrate is subsequently subjected to a substantial thermal treatment or heat history which remelts the polymeric coating, film or laminating material, pinholes often form in the coating, film or laminating material and remain after the substrate is permitted to cool and the polymeric coating, film or laminating material recrystallizes or resolidifies.
Pinholes that form after an initial polymeric coating, film or laminating material is remelted typically form due to volatilization of volatile substances. Volatile substances can profusely contribute to pinholing when they are initially entrained in the substrate or trapped at the interface between the polymeric coating, film or laminating material and the substrate. Volatile substances may be present in the coated or laminated substrate due to contamination occurring during the initial coating or lamination process step or more typically results from excessive moisture contents in the coating material or the substrate. However, to insure proper machining such as folding, cutting and stamping, paperboard substrates are typically specified to contain a minimum of about 6 weight percent moisture. As such, elimination of pinholing by rendering the paperboard substantially dry or by substantially controlling the moisture content of the paperboard is not considered to be a viable option for providing a pinhole-free, pinhole-resistant coated or laminated paperboard.
One example where pinholing can cause performance failures (e.g., leakers) pertains to disposable hot beverage containers made from extrusion coated or laminated paperboard. Known low density polyethylene paperboard coatings and laminates typically provide about 1.5 pinholes per cm.sup.2 of sealing area per 237 cm.sup.3 cup size (e.g. disposable coffee cup service). The pinholes develop and remain after a heat or flame sealing process step in the cup-making operation which is performed to make the back seal and to attach the cup bottom portion to the side walls or cup body.
Occasionally, a coated or laminated paperboard container may still be suitable for a particular service although it although contains numerous pinholes. However, containers intended for hot beverage service (e.g., hot coffee and hot tea) must be substantially pinhole-free, otherwise leakage will occur and/or the paperboard itself will become saturated and structurally weakened.
Notwithstanding the various known olefin polymer extrusion compositions, there remains a need for an olefin polymer extrusion composition which provides less than 1.5 pinholes per cm.sup.2 of sealing area when tested on a 237 cm.sup.3 coated or laminated paperboard cup having a back and bottom portion seals wherein the seals are made using sealing temperatures above the softening point of the composition. There is also a need for a method of making pinhole-free, pinhole resistant extrusion coatings, extrusion cast films, and blown films. There is also a need for a pinhole-free, pinhole-resistant coated or laminated article such as paperboard (or any other substrate generally characterized as having more than 1 weight percent volatile substance content). There is also a need for a method of making a pinhole-free, pinhole-resistant container for flowable materials. There is also a need for a method of making a pinhole-free, pinhole-resistant disposable container. There is a need for a method of making a pinhole-free, pinhole-resistant coated or laminated article such as a paperboard beverage cup.